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All communication from user to kernel can happen very efficiently. With such a model, we have eliminated any memory copies and locks entirely. And a completion queue (CQ), where the kernel is the producer pushing completion results, and the user consumes them. A submission queue (SQ), where the user acts as the producer and pushes syscall requests, and the kernel consumes them. And as you might have already figured out, we need two ring buffers. A ring buffer allows efficient synchronization between producers and consumers with no locking at all.

And once it’s done, it signals the user that it’s ready, and the user accepts them.įortunately, there’s an age-old solution to this problem - ring buffers. The user creates syscall requests, the kernel accepts them. If we look at the problem as the user and kernel being two separate components talking with each other - it is essentially a producer-consumer problem. Otherwise we encounter the same problem - because the application would need to make a syscall again to perform the locking. That takes care of copying, but what about synchronization? Even if we don’t copy, we need some way to synchronize data access between us and the kernel. Ok, that can be done using the mmap system call which can map a chunk of memory that is shared between the user and kernel. How do we prevent copies happening from user-space to kernel-space? Well, obviously we need some sort of shared memory. Let’s think of the first problem at hand - copies. This is great, but it still does not solve the main problem - which is that copies still happen, and the actual syscall still blocks.

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This is how Go gives the appearance of “non-blocking”-ness when your code enters a system call.

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If it cannot get a free P, it just pushes the goroutine to a queue to be executed later, and stores the thread in a pool. And when the syscall returns, the runtime tries to re-attach it to a P. Whenever we make a blocking syscall, the runtime is aware of it, and it detaches the thread from the P executing the goroutine, and acquires a new thread to execute other goroutines. This level of indirection allows it to do some interesting optimizations. It uses a virtual entity (commonly referred to as P) which contains a queue of goroutines to run, which is then mapped to OS threads. Go handles this problem by bringing another layer between the application and the kernel - the runtime. Right off the bat, we can see a lot of bottlenecks - there’s lots of copying going around, and there’s blocking. All this while, the syscall remains blocked. After kernel is done with the execution, it copies the result back to user-space buffers. We make a syscall, our application in user layer calls into the kernel, it makes a copy of the data in kernel space. Let’s take a step back and think of how usual syscalls work.

And then we will construct a small demo application with Go to play with it. Let’s dive into it and try to understand what it is and how it solves the problem. And rightly so, because it gives us an entirely new model to interact with the kernel. There’s a lot of buzz happening around it. And in buffered mode, it would behave synchronously.Īll that is changing slowly, because now we have a brand new interface to perform I/O with the kernel - io_uring. Not a very good way to do I/O when we are trying to make things go fast. This will make the kernel bypass any OS caches and try to read/write from/to the device directly. The major reason being that it only works if the file is opened with the O_DIRECT flag.

We’ve had async_io for a while now, but apart from a small niche set of applications, it isn’t very beneficial. But in the filesystem I/O department, things were a bit lacking. For network I/O, we’ve had the epoll family of syscalls which have provided us with reasonably fast performance.

And especially in a post-Spectre/Meltdown world, this is all the more important.Ī major chunk of the syscalls deal with I/O, because that’s what most applications do. Therefore, it is vital that they are fast. They are the primary interface through which an application interacts with the kernel. In Linux, system calls (syscalls) are at the heart of everything.

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